H. Uchiyama

Positron annihilation studies of icosahedral quasicrystals and their approximants in the Al?Cu?Ru?(Si) alloy systems

The positron lifetimes for the icosahedral quasicrystal Al62.4Cu25.4Ru12.2 and its cubic approximants (1/ 1-Al58Cu31.5Ru10.5, 1/ 1-Al68Cu7Ru17Si8, and 1/0-Al55Cu15Ru20Si10), two-detector coincident Doppler broadening for the icosahedral quasicrystal Al62.4Cu25.4Ru12.2 and its 1/ 1-Al68Cu7Ru17Si8 cubic approximant, and the Doppler broadening obtained by making use of a slow positron beam for the 1/ 1-Al58Cu31.5Ru10.5 cubic approximant have been measured. Structurally intrinsic trapping sites giving rise to saturation trapping were detected by lifetime measurements. The chemical environments of the trapping sites in the icosahedral quasicrystal Al62.4Cu25.4Ru12.2 and the 1/ 1-Al68Cu7Ru17Si8 cubic approximant were determined by coincident Doppler broadening techniques to be dominantly surrounded by Al atoms. The positron diffusion length in the 1/ 1-Al58Cu31.5Ru10.5 cubic approximant derived from the measured S parameter measured by means of a slow positron beam was ??, which is clearly too short, probably due to the high concentration of trapping sites as described above. The atomic structures of the icosahedral quasicrystal Al62.4Cu25.4Ru12.2 and its variety of approximants are discussed and compared to the present proposed model.

The study of 1/1-AlCuRuSi approximate phase by positron annihilation method

Abstract We have measured positron-annihilation lifetime spectra and coincidence Doppler broadening spectra in a 1/1-Al 68 Cu 7 Ru 17 Si 8 approximate phase. These measurements indicate that the 1/1-AlCuRuSi approximate phase contains a dense distribution of structural vacant-type sites. Furthermore, it is found that the first shell around the vacant site is composed almost of Al or Si atoms.

Positron diffusion in icosahedarl quasicrystals

We have analyzed the measured S-parameters as a function of positron incident energy for the icosahedral quasicrystal (QC) Al 71.5 Pd 20.3 Mn 8.2 and Al 70.7 Pd 21.34 Re 7.96 within the framework of the structural model of approximant crystal. Highly resistive icosahedral QC Al 72 Pd 21 Re 7 was investigated by the slow positron beam technique. The result indicated that icosahedral QC Al 72 Pd 21 Re 7 contains a dense distribution of structural vacancy-type sites like another icosahedral QCs. Considerably high structural vacancy densities (on the order of 10 20 cm -3 ) were determined.

Stable quasicrystals studied by means of the slow positron beam

Abstract We have analyzed the measured S-parameter versus the positron incident energy for the icosahedral quasicrystal (QC) Al71.5Pd20.3Mn8.2 by means of the structure of β-(AlPdMnSi) approximate phase. The positron-annihilation Doppler-broadening spectra by variable monoenergetic slow positron beam in the icosahedral QC Al62.5Cu25.0Fe12.5 were measured. This measurement indicates that the icosahedral QC AlCuFe contains a dense distribution of structural vacant sites such as vacant cluster centers.

Positron-trapping property in Al–Mg–Zn alloy system

Abstract Rhombic triacontahedral-type icosahedral quasicrystal and its 1/1- and 2/1-approximants in an Al–Mg–Zn alloy system were systematically studied by means of positron-annihilation measurements. The structural vacancy densities, average positron diffusion length, average positron diffusion length in the vacancy-free region, positron diffusion coefficient, and positron mobility were successfully derived. The results indicated that the thermarized positrons in the vacancy-free region tend to localize as the order of the approximant increases.